TY - JOUR A1 - Farkas, Márton Pál A1 - Hofmann, Hannes A1 - Zimmermann, Günter A1 - Zang, Arno A1 - Bethmann, Falko A1 - Meier, Peter A1 - Cottrell, Mark A1 - Josephson, Neal T1 - Hydromechanical analysis of the second hydraulic stimulation in well PX-1 at the Pohang fractured geothermal reservoir, South Korea JF - Geothermics : an international journal of geothermal research and its applications N2 - In this study, we investigate numerically the hydro-mechanical behavior of fractured crystalline rock due to one of the five hydraulic stimulations at the Pohang Enhanced Geothermal site in South Korea. We use the commercial code FracMan (Golder Associates) that enables studying hydro-mechanical coupled processes in fractured media in three dimensions combining the finite element method with a discrete fracture network. The software is used to simulate fluid pressure perturbation at fractures during hydraulic stimulation. Our numerical simulation shows that pressure history matching can be obtained by partitioning the treatment into separate phases. This results in adjusted stress-aperture relationships. The evolution of aperture adjustment implies that the stimulation mechanism could be a combination of hydraulic fracturing and shearing. The simulated extent of the 0.01 MPa overpressure contour at the end of the treatment equals to similar to 180 m around the injection point. KW - Enhanced Geothermal System KW - Pohang geothermal reservoir KW - hydraulic KW - stimulation KW - PX-1 KW - FracMan Y1 - 2021 U6 - https://doi.org/10.1016/j.geothermics.2020.101990 SN - 0375-6505 SN - 1879-3576 VL - 89 PB - Elsevier CY - Amsterdam [u.a.] ER - TY - JOUR A1 - Hofmann, Hannes A1 - Zimmermann, Günter A1 - Farkas, Márton Pál A1 - Huenges, Ernst A1 - Zang, Arno A1 - Leonhardt, Maria A1 - Kwiatek, Grzegorz A1 - Martinez-Garzon, Patricia A1 - Bohnhoff, Marco A1 - Min, Ki-Bok A1 - Fokker, Peter A1 - Westaway, Rob A1 - Bethmann, Falko A1 - Meier, Peter A1 - Yoon, Kern Shin A1 - Choi, JaiWon A1 - Lee, Tae Jong A1 - Kim, Kwang Yeom T1 - First field application of cyclic soft stimulation at the Pohang Enhanced Geothermal System site in Korea JF - Geophysical journal international N2 - Large-magnitude fluid-injection induced seismic events are a potential risk for geothermal energy developments worldwide. One potential risk mitigation measure is the application of cyclic injection schemes. After validation at small (laboratory) and meso (mine) scale, the concept has now been applied for the first time at field scale at the Pohang Enhanced Geothermal System (EGS) site in Korea. From 7 August until 14 August 2017 a total of 1756 m(3) of surface water was injected into Pohang well PX-1 at flow rates between 1 and 10 l s(-1), with a maximum wellhead pressure (WHP) of 22.8 MPa, according to a site-specific cyclic soft stimulation schedule and traffic light system. A total of 52 induced microearthquakes were detected in real-time during and shortly after the injection, the largest of M-w 1.9. After that event a total of 1771 m(3) of water was produced back from the well over roughly 1 month, during which time no larger-magnitude seismic event was observed. The hydraulic data set exhibits pressure-dependent injectivity increase with fracture opening between 15 and 17 MPa WHP, but no significant permanent transmissivity increase was observed. The maximum magnitude of the induced seismicity during the stimulation period was below the target threshold of M-w 2.0 and additional knowledge about the stimulated reservoir was gained. Additionally, the technical feasibility of cyclic injection at field scale was evaluated. The major factors that limited the maximum earthquake magnitude are believed to be: limiting the injected net fluid volume, flowback after the occurrence of the largest induced seismic event, using a cyclic injection scheme, the application of a traffic light system, and including a priori information from previous investigations and operations in the treatment design. KW - Cyclic soft stimulation (CSS) KW - induced seismicity KW - risk mitigation KW - enhanced geothermal systems (EGS) KW - granite KW - Pohang (Korea) Y1 - 2019 U6 - https://doi.org/10.1093/gji/ggz058 SN - 0956-540X SN - 1365-246X VL - 217 IS - 2 SP - 926 EP - 949 PB - Oxford Univ. Press CY - Oxford ER - TY - JOUR A1 - Farkas, Márton Pál A1 - Yoon, Jeoung Seok A1 - Zang, Arno A1 - Zimmermann, Günter A1 - Stephansson, Ove A1 - Lemon, Michael A1 - Danko, Gyula T1 - Effect of foliation and fluid viscosity on hydraulic fracturing tests in mica schists investigated using distinct element modeling and field data JF - Rock Mechanics and Rock Engineering N2 - Several hydraulic fracturing tests were performed in boreholes located in central Hungary in order to determine the in-situ stress for a geological site investigation. At a depth of about 540m, the observed pressure versus time curves in mica schist with low dip angle foliation shows atypical pressure versus time results. After each pressurization cycle, the fracture breakdown pressure in the first fracturing cycle is lower than the refracturing or reopening pressure in the subsequent pressurizations. It is assumed that the viscosity of the drilling mud and observed foliation of the mica schist have a significant influence on the pressure values. In order to study this problem, numerical modeling was performed using the distinct element code particle flow code, which has been proven to be a valuable tool to investigate rock engineering problems such as hydraulic fracturing. The two-dimensional version of the code applied in this study can simulate hydro-mechanically coupled fluid flow in crystalline rock with low porosity and pre-existing fractures. In this study, the effect of foliation angle and fluid viscosity on the peak pressure is tested. The atypical characteristics of the pressure behaviour are interpreted so that mud with higher viscosity penetrates the sub-horizontal foliation plane, blocks the plane of weakness and makes the partly opened fracture tight and increase the pore pressure which decreases slowly with time. We see this viscous blocking effect as one explanation for the observed increase in fracture reopening pressure in subsequent pressurization cycles. KW - Hydraulic fracturing KW - Stress measurement KW - Particle flow code KW - Hydro-mechanical coupling KW - Microcrack KW - Viscous blocking Y1 - 2019 U6 - https://doi.org/10.1007/s00603-018-1598-7 SN - 0723-2632 SN - 1434-453X VL - 52 IS - 2 SP - 555 EP - 574 PB - Springer CY - Wien ER -